In SE-B-469 466 a two-step electro filter is described, said filter having an ionizing section that downstream, seen in the desired air transport, is followed by a so called electrostatic precipitator. The electrode elements of the electrostatic precipitator, said elements in the disclosed design being planar plates, are preferably made out of a material that is considered to be highly resistive or antistatic (so called dissipative material). However, in other embodiments the electrode elements may have other shapes. By using such a material according to the mentioned patent document an essential improvement of the separation ability may be achieved. The reason therefore is that the voltage that is created between the electrode elements may assume higher values than previously known electrofilters having electrode elements made out of metallic material and hence low resistive material.
In SE-A-9303894-1 a further development of the design of an electrostatic precipitator according to the patent document mentioned above is described. According to SE-A-9303894-1 electrical screening of certain portions of the electrodes of the electrostatic precipitator gives rise to possibilities to further increase the voltage between said electrodes. The effect of this is an increased separation ability.
The present invention is preferably applied in connection with two-step electrofilters according to the patent documents mentioned above. However, the invention can also be used in connection with previously known two-step electrofilters or devices based on ion wind of the type described in EP-B-0 264 363 or other devices for separating electrically charged aerosols (see SE-A-9400110-4).
The electrostatic precipitator of an electrofilter normally consists of a group of electrically repelling electrode elements R and a group of electrically attracting electrodes A arranged in such a way that the electrode elements of one group are positioned with a gap distance (a) and alternately to electrode elements of the other group, said elements of different groups being at different potential vis-a-vis each other.
As described e.g. in SE-B-469 466 said electrode elements are preferably made out of cellulose based material.
In practice it has shown that the performance of the device is to a great extent depending from inter alia paper quality and also external factors as the humidity of the air and the fouling of the filter cassette. By increasing humidity the resistivity of the cellulose material decreases dramatically and a properly functioning and dimensioned electrostatic precipitator loses or reduces its ability to separate particles. Further as a result of the decreased resistivity in the material of the electrodes and a high voltage between said electrodes the electrostatic precipitator may emit substantial amounts of ozone (as a result of the corona discharge primarily between the edge portions of the electrode elements A and R).